Process of producing castings of magnesium and high grade magnesium alloys



Patented Sept. 29, 1931 UNITED STATES PATENT OFFICE ALBERT LEVY MOND, OF LONDON, ENGLAND, ASSIGNOR TO I. FARIBENINDUSTR IE AKTIENGESELLSCHAFT, OF FRANKFORT-ON-THEMAIN, GERMANY PROCESS OF PRODUCING OASTINGS 0F MAGNESIUM AND HIGH MAGNESIUM ALLOYS No Drawing. Application filed December 14, 1929, Serial No. 414,24Q, and in Great Britain December This invention relatesto the art of making castings of magnesium and high grade magnesium alloys.

' An object ofthe present invention is to 6 provide a method for producing-castings of the aforesaid metals in green sand moulds i. e. unbaked moulds still containing, the moisture requisite to render the, sand mouldable. 1 Another object is to obtain castings formed on the castings incidentally with the casting processresistant to corrosion in a higher degree than hitherto feasible. Such protective layers are known per se, but their formation required a special treatment, i. e. a distinct operation entirely independent of the casting process. Other objects of the invention will appear from the following.

In U. S. A. Patent 1,463,609, a process of castingmagnesium and magnesium alloys is described which consists in applying to the mould, prior to casting the metal, a substance which is capable of giving off, in contact with the molten metal, a gaseous substance which is more easily oxidizable than magnesium. In U. S. A. Patents 1,584,072 and 1,614,820, an improved modification of this process is described, inasmuch as the moulds are prepared from a mixture of green sand with nonmetallic substance incapable of reaction with the metal and adapted, when heated by the heat emitted from the cast metal, to form on the surface of the casting a layer preventing the casting from being spoilt by the access of air and water vapour. As suitable substances of this kind are mentioned sulphur, boric acid, oxalic acid compounds, bicarbonates, urea- I and mixtures of these substances. It will be noted that the substances thus claimed are the metal and it further appears that the protective layer on the metal is only present during the casting operation, generally in the which are-by virtue of a protective layer supposed to be incapable of reaction with ,ation of ammonia, during casting when using moulding sand containing urea and as we have actually observed the presence of am. monia we tried other ammonium compounds for the same purposes.

It is further known that the resistance of articles made of magnesium and magnesium alloys to corrosion is, by asuperficial treatment with fluoride solutions, considerably increased, probably owing to the formation of solidifying metal. However, the hygroscopic properties of ammonium fluoride provided'to be a greatdisadvantage, inasmuch as ammonium fluoride in contact with sand and water or moisture, readily decomposes giving off ammonia vapours. The moulds thus rapidly become impoverished with respect to the protective substance and this detrimental effect is still further promoted by the heating of the sand in the courseof casting. The sand therefore practically can not be used over again without readdition of ammonium fluoride.

A further object of the present invention istherefore, to avoid the detrimental effects of such hygroscopicity in the material constituting the mould.

This object can be fulfilled in numerous difl'erent ways. It may be either achieved by embodying in the sand, together with amsmaller tendency to decompose under the influence of moisture and heat. Thus, according to one embodiment of.the

invention, I employ a fluorine compound which is non-hygroscopic and which at the same time has the property, when coming into contact with the molten metal, of creating a neutral atmosphere and of providing a protective layer on the cast metal.

I have found that a suitable substance of this kind is the acid ammonium fluoride or ammonium hydrofluoride NH F. HF, which when mixed with green sand in amounts preferably ranging between 3 and 5 per cent ensures perfect castings. As practically no decomposition of the salt occurs the sand may be repeatedly used without further addition of this protective ingredient..

By way of example I have further found that a substance which in contact with water yields a non-hygroscopic fluoride compound by decomposition, namely ammonium silicoflouride, when admixed to the sand in the aforementionedproportions, also gives satisfactory results.

The property of this salt of decomposing, in the presence of water, to form a non-hygroscopic fluoride offers the advantage that by its employment the free moisture of the green sand is simultaneously reduced, i. e. part of the moisture of the sand is withdrawn. This is probably due to the fact that an equilibrium is established between ammonium silico-fluoride and water, which can be explained by the following equation:

ple experiment of dissolving ammonium sil1- 1n co-fluoride in water, when after a short while silicic acid is precipitated, which is capable of absorbing water. In addition free hydrofluoric acid is formed. When using ammonium silico-fluoride in practice it is advantageous, before forming the mould, to store the sand to which the silico-fluoride has been admixed, preferably in a quantity amounting to about from 3 to 5 per cent, for some time, until the reaction has taken place between the ammonium silico-fluoride and the.

water present in the green sand and until the silica gel formed has absorbed some of the moisture in the green sand.

If this precaution be omitted the sand will swell owing to the increase in volume of the silica gel and if such sand be used immediately the impression of the pattern will become distorted. The free hydrofluoric acid 4 or other fluorine compounds that may be produced during casting have the property of forming an anti-corrosive layer on the metal itself.

According to a modification of the invention, I may use a hygroscopic fluorine compound such as ammonium fluoride and associate therewith a substance adapted to obviate the detrimental effects of the hygroscopicity of such compound, for example by combining or absorbing or adsorbing the decomposition products.

I have found that oxalic acid, when added to a sand containing the hygroscopic ammonium floride, very efficiently reduces the decomposition of the latter in contact with the moisture of the sand. Thus a green sand thoroughly mixed with about 5 percent of ammonium fluoride and about 3 percent of oxalic acid shows hardly any decomposition and is suitable for casting magnesium and magnesium alloys, at the same time providing the finished castings with a protective layer of a fluoride compound.

I have further found that the tendency of the ammonium silico-fluoride to cause a swelling of the moist moulding sand, thus necessitating a storing of the sand before using it for moulding, is also substantially checked by incorporating, in the sand, oxalic acid, preferably also in a quantity of about 3 percent, together with ammonium silicofluoride. In this case, a storing of the sand before use is no longer necessary.

The substances and compositions hereinbefore described, may in the case of small castings be employed by themselves in amounts preferably not exceeding 15 percent. In the case of larger castings, however, it is frequently advisable to further add one or more of the protective substances disclosed in the specification of the U. S. A. Patents 1,463,609, 1,584,072 and 1,614,820, preferably small percentages of sulphur and/or boric acid, to the sand, so as to efliciently prevent local scorch- It will be understood that the application of the substances hereinbefore described is not limited to impregnating the sand by admixing these substances, although this is generally the most economical and simple way of applying them. Any mode of application which will ensure contact between the molten metal and the protective substances disclosed within the mould falls within the scope of the present invention. Thus, for example, it is also sufficientto impregnate the sand constituting the inner surface of the finished mould with the substances named before casting the metal into the mould.

I claim:

' 1. A process of producing castings of magnesium and high grade magnesium alloys which comprises bringing ammonium fluoride and an acid capable of absorbing ammonia into contact with the surface of the molten metal within the mould.

2., A process of producing castings of magnesium and high grade magnesium alloys which comprises bringing ammonium fluoride and an acid of the group consisting of hydrofluoric acid and oxalic acid into COH- tact with the surface of the molten metal within the mould.

3. The method of making green sand moulds for easing a magnesium and magnesium alloys which comprises impregnating at least the inner surface of the mould with am- 6 monium fluoride and an acid capable of absorbing ammonia.

4. The method of making green sand moulds for casting magnesium and magnesium alloys which comprises impregnating at 10 least the inner surface of the mould with ammonium fluoride and an acid of the group condsisting of hydrofluoric acid and oxalic 5. A process of producing castings of magnesium and high grade magnesium alloys which comprises bringing an inert protective substance and ammonium fluoride and an acid capable of absorbing ammonia into contact with the surface of the molten metal within the mould. I

6. A process of producing castings of magnesium and high grade magnesium alloys which comprises bringing sulphur and 1 ammonium fluoride and an acid of the group 26 consisting of hydrofluoric acid and oxalic acid into contact with the surface of the molten metal within the mould. 1

7. The method of making green sand moulds for casting magnesium and magnesio ium alloys which comprises impregnating at least the inner surface of the mould with an inert protective substance and. ammonium fluoride and an acid capable of absorbing ammonia.

- 8. The method of making green sand moulds for casting magnesium and magnesium alloys which comprises impregnating at least the inner surface of the mould with sulphur and ammonium fluoride and an acid goof the group consisting of hydrofluoric acid and oxalic acid.

' In testimony whereof I have hereunto set my hand.

ALBERT LEVY MOND. 

